1. Field of the Invention
The present invention relates to an aromatic hydrocarbon production apparatus for obtaining aromatic hydrocarbons from a feedstock, such as reformate, containing aromatic hydrocarbons.
2. Description of the Related Art
Among aromatic hydrocarbons, benzene and xylene are significant basic chemicals used as raw materials for various industrial products. Benzene is mainly used for styrene monomer derivatives serving as raw materials for polystyrene or an ABS resin. Para-xylene, amongst xylenes, is used as a raw material for polyester fiber/resin and is one of basic chemicals in greatest demand.
An aromatic hydrocarbon production apparatus (also referred to as an aromatics complex) includes an aromatics extraction apparatus for separating benzene from a feedstock, such as reformate or pyrolysis gasoline, containing aromatic hydrocarbons and a para-xylene production apparatus for separating para-xylene from the feedstock. The aromatics complex is provided with a large number of distillation columns. Distillation is a unit operation that consumes large energy, and energy consumption is large in production of aromatic hydrocarbons.
In order to reduce the energy consumption, a multiple effect method is conventionally employed among distillation columns provided in an aromatics complex. Typically, an operation pressure of a xylene column, whose column bottom temperature is essentially so high that a heating furnace is necessary, is increased so that overhead vapor of the xylene column can be used as the heat source of a reboiler of another distillation column. Since energy necessary for the xylene column per se is essentially large, however, even if the energy is saved by using the overhead vapor of the xylene column as the heat source of a reboiler of another distillation column without reducing the energy necessary for the xylene column per se, there is a limit in reducing the energy consumption of the whole process.
In techniques described in US 2012/0048711A1 and US 2012/0048718A1, the energy is saved by using, as a xylene column, two distillation columns, that is, a low-pressure xylene column and a high-pressure xylene column. These literatures disclose a technique in which overhead vapor of the high-pressure xylene column is used as the heat source of a reboiler of the low-pressure xylene column and also as the heat source of a reboiler of another distillation column.
On the other hand, JP H08-66601A, JP 2004-16928A and International Publication No. WO2011/043199 disclose a heat integrated distillation column (hereinafter sometimes referred to as “HIDiC”) capable of reducing energy consumption in distillation. In the HIDiC, heat is transferred by heat exchange from a rectifying section (a section located above a feedstock feed position) of a distillation column to a stripping section (a section located below the feedstock feed position), so as to reduce the amount of heat supplied to a reboiler and the amount of heat removed at a condenser, and thus, the thermal efficiency is improved.